1. Field of the Invention
This invention relates to .alpha.-allenic-.alpha.-amino acids which are useful as enzyme inhibitors of the suicide or k.sub.cat type.
2. Background Art
Suicide enzyme inhibitors are substances bearing a latent reactive group that is unmasked by the target enzyme itself and, upon being unmasked, reacts with the enzyme in an irreversible manner, thus inactivating it.
A number of enzyme inhibitors of the suicide type are known in the art, see for example the reviews by Walsh, Horizons in Biochem., Biophys., 3, 36-81 (1977) and Jung, M. J., and J. Koch-weser, "Molecular Basis of Drug Action," Singer and Ondarza, Ed., 135-150 (1981), Elsevier North Holland Inc.
The most common prosthetic group of target decarboxylases is normally pyridoxal-5'-phosphate. However, two enzymes are known in which the prosthetic group is a pyruvoyl residue attached to the protein chain: mammalian S-adenosyl L-methionine-decarboxylase and bacterial histidine decarboxylase. The mechanism of action of the pyridoxal-dependent enzymes can be summarized as follows: the .alpha.-amino acid enters the enzyme's active site and forms a Schiff-base with the aldehyde function of the cofactor already bound to the enzyme. Carbon dioxide is then eliminated, generating a negative charge on the .alpha.-carbon which can be delocalized over the whole pyridine nucleus of the cofactor. Protonation usually takes place on the .alpha.-carbon atom giving, after hydrolysis, the corresponding amine and pyridoxal phosphate. It has been suggested that in pyridoxal catalysis, the .alpha.-carbon bond to be broken must lie in a plane perpendicular to the plane of the cofactor-amine system in order to minimize the energy of the transition state. One role of the enzyme is, therefore, to freeze the conformation of the amino acid-pyridoxal adduct in this special arrangement.
The process of decarboxylation and protonation appears to be stereo-specific and occurs with retention of configuration. The mechanism of decarboxylation catalyzed by the pyruvoyl-dependent enzymes is presumably similar, the carbonyl group of the pyruvate replacing the aldehyde function of pyridoxal-5'-phosphate.
Various types of chemical modifications of the substrate likely to generate enzyme-activated irreversible inhibitors have been synthesized and investigated. For example, replacement of the .alpha.-hydrogen by a vinyl or ethynyl group can generate an .alpha., .beta.-unsaturated imine, a good Michael acceptor. A number of .alpha.-vinyl or .alpha.-ethynyl substituted amino acids had been prepared and tested as suicide inhibitors of decarboxylase enzymes. This concept has also been tested by incorporation of the double bond directly into the amino acid chain rather than as a replacement of the .alpha. hydrogen. It has been found that .beta., .gamma.-dehydroornithine is a very potent competitive inhibitor of ornithine decarboxylase. See Relyea, N and R. R. Rando, Biochem. Biophys. Res. Comm., 67:292-402 (1975). Also it has been found that .alpha.-methyl-trans-dehydroglutamic acid irreversibly inhibits rat brain glutamate decarboxylase, Crystal, E., et al, J. Neurochem., 32:1501-1507 (1977).
The amine analogs of the .alpha.-halomethyl substituted and .beta., .gamma.-unsaturated amino acids have also been tested as decarboxylase enzyme inhibitors. Not unsurprisingly, several compounds from both of these classifications have been found to be active in several systems. For example (R) 4-amino-hex-5-ynoic acid has been found to inhibit bacterial and mammalian glutamate decarboxylase, Jung, M. J., et al, Biochem., 17:2628-2632 (1978). The compound (-)-5-hexyne-1,4-diamine has been found to be a potent time-dependent inhibitor of rat liver and rat prostate ornithine decarboxylase. See Metcalf, B. W., et al, J. Amer. Chem. Soc., 100:2551-2553 (1979), .alpha.-ethynyl- and .alpha.-vinyl-dopamine cause a slow-time dependent inactivation of aromatic amino acid decarboxylase. See Maycock, A. L., et al, "Drug Action and Design: Mechanism-Based Enzyme Inhibitors, "Elsevier, North Holland, pp. 115-129.
It is the object of this invention to provide a group of novel .alpha.-allenic substituted amino acids wherein the .beta., .gamma.-unsaturation provides a reactive site which is capable of undergoing an irreversible action with the enzyme target thus abolishing decarboxylase activity.